That’s one of the shocking new theories being drawn from a series of anthropology papers published Friday in a special edition of the journal Science.
Scientists say a 4.4-million-year-old fossil called Ardi – short for ardipithecus ramidus – is descended from the “missing link,” or the last common ancestor between humans and apes.
The 4-foot, 110-pound female’s skeleton and physiological characteristics bear a closer resemblance to modern-day humans than to contemporary apes, meaning they evolved from human-like creatures – not the other way around.
The partial skeleton “is probably the most important find we have had yet,” says Owen Lovejoy, one of the primary authors on the journal package.
“It’s transformative. This is a lot closer to anything that you’d call the missing link than anything that’s ever been found,” says Lovejoy, an anthropologist at Ohio’s Kent State University.
Among other things, research on Ardi suggests humans are far more primitive in an evolutionary sense than today’s great apes – like chimps and gorillas – which have continued to evolve from the missing link.
“In a way we’re saying that the old idea that we evolved from a chimpanzee is totally incorrect,” he says. “It’s more proper to say that chimpanzees evolved from us.”
Lovejoy says chimps experienced more profound evolutionary changes in their backs, pelvises, limbs, hands and feet as they adapted to life in the trees than the hominid line of upright species that evolved into humans.
“Hominids, it turns out to be, are pretty primitive,” he says.
Lovejoy explains the actual missing link – or last common ancestor in scientific parlance – may have first sprung up some six million years before Ardi.
“It’s the first find that we have that is really informative about what that last common ancestor was like.”
Along with busting the supposed ape-to-man lineage, Lovejoy says, Ardi has obliterated existing theories about how, where and why our ancestors began the signature practice of upright walking.
First off, he says, none of our ancestors were like the “knuckle walkers” shown at the start of those famous human decent lineups.
The loping, knuckle-down gait that characterizes gorillas on the ground comes as a result of the stiffened backs they evolved to help in their arboreal acrobatics.
Hominids like Ardi, Lovejoy says, had pliant lower backs that allowed them to stand upright. And while they would often have moved palms-down across the ground like monkeys, they were bipedal much of the time.
But why did Ardi and her kin evolve this pedestrian practice in the first place?
Traditional thinking, Lovejoy says, is that our distant ancestors first stood up to get a better view of lurking predators and potential prey after they had knuckle-walked their way out of the forest and onto the grassy savannahs of Africa.
But Ardi, as two of the journal papers show, walked tall in a wooded setting, where trees would have negated the improved sightlines of added height.
Found in the Afar Triangle area of Ethiopia in 1992, Ardi’s incomplete skeleton was pieced together from about 100 bone fragments that took three years to fully uncover. While the area is now desert, it was semideciduous woodland when the creature lived, says Kathlyn Stewart, a scientist with Ottawa’s Canadian Museum of Nature, who collaborated on one of these papers.
Go back 4.4 million years and you would find Ardi’s kind climbing in the trees and foraging upright on the ground, their feet planted comfortably in both worlds.
“This thing is both arboreal and terrestrial,” Lovejoy says. “Its pelvis is kind of a mosaic, the upper part is adapted to upright walking, but the lower part is adapted to climbing trees.”
Unlike the earliest previously known pre-human – a creature known as Lucy, who lived 1.2 million years later – Ardi still had the opposable big toe of a tree climber.
Yet the males of her species also had the small canine teeth that distinguish humans from great apes.
This dental detail, says Lovejoy, is a key clue to the walking mystery.
Male apes with small canines were less capable of fighting off competitors, Lovejoy says, and would have to offer females something more for mating favours.
“Instead of males gaining access to females by threatening other males … they’re getting access to females by providing them food,” he says. Upright walking made it easier to carry that food through their woodland environment.
“So the whole savannah theory (of walking) is now gone as well.”
In an independent analysis of the research, Alan Walker, a Pennsylvania State University paleoanthropologist, called the Ardi fossils “extraordinary.”
“The anatomy behind this behavioural combination is very unexpected and is certain to cause considerable rethinking of not only our evolutionary past, but also that of our living relatives, the great apes.”